Ferritic stainless steel sheet and method for manufacturing the same

ABSTRACT

Provided are a ferritic stainless steel sheet and a method for manufacturing the steel sheet. The ferritic stainless steel sheet according to the present invention has a chemical composition containing, by mass %, C: 0.005% to 0.025%, Si: 0.02% to 0.50%, Mn: 0.55% to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001% to 0.10%, Cr: 15.5% to 18.0%, Ni: 0.1% to 1.0%, N: 0.005% to 0.025%, and the balance being Fe and inevitable impurities, an elongation after fracture of 28% or more, an average r-value of 0.75 or more, and a minimum value of maximum logarithmic strain at the forming limit, which is, determined on the basis of a forming limit diagram (FLD), of 0.15 or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2015/003339, filed Jul. 2, 2015, the disclosure of this application being incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a ferritic stainless steel sheet having sufficient corrosion resistance and being excellent in formability and ridging resistance and a method for manufacturing the steel sheet.

BACKGROUND OF THE INVENTION

A ferritic stainless steel sheet is more economical than austenitic stainless steel, which contains a large amount of expensive Ni. Since a SUS430-type stainless steel sheet (containing 16 mass % to 18 mass % of Cr) is an especially economical ferritic stainless steel, it is used for various applications such as building materials, transporting instruments, home electrical appliances, kitchen appliances, and automobile parts, and its range of applications is being further expanded nowadays. A material to be used in such applications is required to have not only satisfactory corrosion resistance but also sufficient formability such that the material can be formed into a specified shape.

On the other hand, since a SUS430-type stainless steel sheet is used in applications in which the steel sheet is required to have a good surface appearance in many cases, the steel is also required to be excellent in terms of ridging resistance. The term “ridging” means surface asperity caused by strain due to forming work. In the case of a ferritic stainless steel sheet, there may be a case where a colony, which is a group of crystal grains having similar crystal orientations, is formed when casting and/or hot rolling are performed. In the case of a steel sheet in which a colony is retained, since there is a large difference in the amount of strain between a portion in which a colony is formed and other portions when forming work is performed, surface asperity (ridging) is generated after forming has been performed. In the case where an excessively large amount of ridging is generated after forming has been performed, since a polishing process is necessary in order to remove the surface asperity, there is a problem of an increase in the manufacturing costs of a formed product.

Patent Literature 1 discloses ferritic stainless steel excellent in terms of formability, the steel having a chemical composition containing, by massa, C: 0.02% to 0.06%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.005% or less, Ti: 0.005% or less, Cr: 11% to 30%, and Ni: 0.7% or less, in which the relationships 0.06 S (C +N) 0.12, 1≤N/C, and 1.5×10⁻³≤(V×N)≤1.5×10⁻² (C, N, and V respectively denote the mass % of the corresponding chemical elements) are satisfied. However, an excellent elongation after fracture in the rolling direction of a steel sheet was achieved when the present inventors manufactured ferritic stainless steel by using the method according to Patent Literature 1. However, when the present inventors attempted to manufacture a ventilation hood by using press working which requires mainly bulge forming capability, it was not possible to obtain a specified shape, which means that it was not possible to achieve such a high level of bulge forming capability as to be expected from the elongation after fracture. Moreover, in the example according to Patent Literature 1, so-called box annealing (for example, annealing at a temperature of 860° C. for 8 hours) is performed after hot rolling has been performed. Since such box annealing takes about one week including heating and cooling processes, there is a problem of a decrease in productivity. In addition, since a technique for decreasing the amount of solid solution N by adding an expensive transition metal element, that is, V, is used, there is also a problem of an increase in manufacturing costs. Moreover, since hot-rolled-sheet annealing is performed by using a box annealing method in a temperature range in which a ferrite single phase is formed, most ferrite colonies are retained without being broken, which also results in a problem of a significant deterioration in ridging resistance.

Patent Literature 2 discloses ferritic stainless steel excellent in terms of workability and surface quality, the steel being manufactured by performing hot rolling on steel having a chemical composition containing, by mass %, C: 0.01% to 0.10%, Si: 0.05% to 0.50%, Mn: 0.05% to 1.00%, Ni: 0.01% to 0.50%, Cr: 10% to 20%, Mo: 0.005% to 0.50%, Cu: 0.01% to 0.50%, V: 0.001% to 0.50%, Ti: 0.001% to 0.50%, Al: 0.01% to 0.20%, Nb: 0.001% to 0.50%, N: 0.005% to 0.050%, and B: 0.00010% to 0.00500%, by performing hot-rolled-sheet annealing on the hot-rolled steel sheet by using a box annealing furnace or a continuous furnace in an AP line (annealing and pickling line) in a temperature range in which a ferrite single phase is formed, and by further performing cold rolling and finish annealing. However, in the case where a box annealing furnace is used, there is a problem of low productivity as in the case of Patent Literature 1 described above. In addition, when the present inventors attempted to manufacture parts by using the steel according to Patent Literature 2 and press working which involves mainly bulge forming as in the case of Patent Literature 1, it was not possible to obtain a specified shape, which means that it was not possible to achieve such a high level of bulge forming capability as to be expected from elongation after fracture. Moreover, generally, in the case of ferritic stainless steel such as that according to Patent Literature 2, since a colony, which is a group of crystal grains having similar crystal orientations, is formed when casting or hot rolling is performed, it is not possiblesto sufficiently break the colony of a ferrite phase in the case where hot-rolled-sheet annealing is performed in a temperature range in which a ferrite single phase is formed. Therefore, the colony is retained in the elongated state in the rolling direction of cold rolling which is performed after hot-rolled-sheet annealing, which results in a problem of significant ridging occurring after forming has been performed.

Patent Literature 3 discloses a method for manufacturing a ferritic stainless steel sheet excellent in terms of ridging resistance and workability, the method including performing annealing in a temperature range of 930° C. to 990° C., in which an austenite phase and ferrite phase coexist, for 10 minutes or less on a hot-rolled steel sheet made of ferritic stainless steel containing 0.15% or less of C and 13% to 25% of Cr in order to form a dual-phase microstructure composed of a martensite phase and a ferrite phase and performing cold rolling and cold-rolled-sheet annealing. Patent Literature 3 mentions only elongation as workability. However, when the present inventors manufactured a steel sheet by using the method according to Patent Literature 3 and attempted to manufacture a ventilation hood by using a method which involves mainly bulge forming, there were some cases where it was not possible to obtain a specified shape due to cracking occurring during press working, which clarifies that there may be a case where it is not possible to achieve such a high level of bulge forming capability as to be expected from its elongation after fracture. As described above, although the ferritic stainless steel sheet according to Patent Literature 3 has a high elongation after fracture in a tensile test, since it is not possible to sufficiently achieve a high level of bulge forming capability, which is required in press forming, it is difficult to say that sufficient formability, which is an issue to be addressed by aspects of the present invention, is achieved.

As described above, a technique for manufacturing a SUS430-type stainless steel sheet having sufficient corrosion resistance and being excellent in formability and ridging resistance has not yet been established.

CITATION LIST Patent Literature

PTL 1: Japanese Patent No. 3584881 (Domestic Re-publication of PCT International Publication for Patent Application No. WO00/60134)

PTL 2: Japanese Patent No. 3581801 (Japanese Unexamined Patent Application Publication No. 2001-3134)

PTL 3: Japanese Examined Patent Application Publication No. 47-1878

SUMMARY OF THE INVENTION

An object of aspects of the present invention is, by solving the problems described above, to provide a ferritic stainless steel sheet having sufficient corrosion resistance and being excellent in formability and ridging resistance and a method for manufacturing the steel sheet.

Here, in accordance with aspects of the present invention, the term “sufficient corrosion resistance” means a case where a rust area ratio (=(rust area)/(total steel sheet area)×100 [%]) in the surface of a steel sheet is 25% or less in the case where 8 cycles of a salt spray cyclic corrosion test are performed in accordance with the prescription in JIS H 8502 on a steel sheet whose surface has been polished by using #600 emery paper and whose end surfaces are sealed, where a unit cycle consists of salt spraying (35° C., 5-massa-NaCl, spraying for 2 hours), drying (60° C., relative humidity 40%, 4 hours), and wetting (50° C., relative humidity 95%, 2 hours).

In addition, the term “excellent formability” means a case where a steel sheet has excellent bulge forming capability, elongation after fracture, and average r value. The term “excellent bulge forming capability” means a case where the minimum value of maximum logarithmic strain at the forming limit, which is determined on the basis of a forming limit diagram (FLD) of steel, is 0.15 or more. The term “excellent elongation after fracture ” means a case where the elongation after fracture (El) of each of the test pieces in the rolling direction and in a direction at a right angle to the rolling direction is 28% or more in a tensile test in accordance with JIS Z 2241. The term “excellent average r value” means a case where an average Lankford value (hereinafter, referred to as “average r-value”), which is calculated by using equation (1) below when a test piece is subjected to a strain of 15% in a tensile test in accordance with JIS Z 2241, is 0.75 or more.

average r−value=(r _(L)+2×r _(d)+r_(c))/4   (1)

Here, r_(L) denotes an r-value determined when a tensile test is performed in a direction parallel to the rolling direction, r_(D) denotes an r-value determined when a tensile test is performed in a direction at an angle of 45° to the rolling direction, and r_(c) denotes an r-value determined when a tensile test is performed in a direction at a right angle to the rolling direction.

Moreover, the term “excellent in terms of ridging resistance” means a case where a ridging height which is determined by using the method described below is 2.5 μm or less. First, in order to determine ridging height, a JIS No. 5 tensile test piece is taken in a direction parallel to the rolling direction. Subsequently, after having polished the surface of the taken test piece by using #600 emery paper, the test piece is subjected to a tensile strain of 20%. Subsequently, arithmetic average waviness (Wa) is determined in accordance with JIS B 0601 (2001) in a direction at a right angle to the rolling direction in the polished surface of the central portion of the parallel portion of the test piece by using a surface roughness meter. The determination is performed under the conditions of an observation length of 16 mm, a wavelength of a high-frequency cutoff filter of 0.8 mm, and a wavelength of a low-frequency cutoff filter of 8 mm. The arithmetic average waviness is defined as ridging height.

From the results of investigations conducted in order to solve the problems, the following knowledge was obtained. By performing annealing (hereinafter, referred to as “hot-rolled-sheet annealing”) in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase can preferably be formed after having performed hot rolling and before performing cold rolling on ferritic stainless steel sheet having an appropriate chemical composition, and by further performing annealing (hereinafter, referred to as “cold-rolled-sheet annealing”) on the cold-rolled steel sheet in a temperature range in which a ferrite single phase is formed, a mixed-grain microstructure composed of ferrite grains containing a large number of carbonitrides and ferrite grains containing a small number of carbonitrides is formed, although it is a ferrite single-phase microstructure. It was found that, as a result, it is possible to obtain a ferritic stainless steel sheet having sufficient corrosion resistance and being excellent in formability and ridging resistance.

Aspects of the present invention have been completed on the basis of the knowledge described above, and the subject matter of aspects of the present invention is as follows.

[1] A ferritic stainless steel sheet having a chemical composition containing, by massa, C: 0.005% to 0.025%, Si: 0.02% to 0.50%, Mn: 0.55% to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001% to 0.10%, Cr: 15.5% to 18.0%, Ni: 0.1% to 1.0%, N: 0.005% to 0.025%, and the balance being Fe and inevitable impurities; an elongation after fracture of 28% or more; an average r-value of 0.75 or more; and a minimum value of maximum logarithmic strain at the forming limit, which is determined on the basis of a forming limit diagram (FLD), of 0.15 or more.

[2] The ferritic stainless steel sheet according to item [1] above, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from Cu: 0.1% to 1.0%, V: 0.01% to 0.10%, Ti: 0.001% to 0.05%, Nb: 0.001% to 0.05%, Mo: 0.1% to 0.5%, and Co: 0.01% to 0.2%.

[3] The ferritic stainless steel sheet according to item [1] or [2] above, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from Mg: 0.0002% to 0.0050%, Ca: 0.0002% to 0.0020%, B: 0.0002% to 0.0050%, and REM: 0.01% to 0.10%.

[4] A method for manufacturing the ferritic stainless steel sheet according to any one of items [1] to [3] above, the method including performing hot rolling on a steel slab, performing annealing in which the hot-rolled steel sheet is held in a temperature range of 900° C. to 1100° C. for 5 seconds to 15 minutes, subsequently performing cold rolling, and performing annealing in which the cold-rolled steel sheet is held in a temperature range of 800° C. to 900° C. for 5 seconds to 5 minutes.

Here, in the present description, % used when describing the chemical composition of steel always means mass %.

According to aspects of the present invention, it is possible to obtain a ferritic stainless steel sheet having sufficient corrosion resistance and being excellent in formability and ridging resistance.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereafter, embodiments of the present invention will be described in detail.

The ferritic stainless steel sheet according to aspects of the present invention is intended to be subjected to press working so as to be used in various applications such as architectural parts, parts of home electrical appliances, kitchen appliances, and automobile parts. A material to be used in such applications is required to have sufficient formability.

However, a technique for manufacturing SUS430-type ferritic stainless steel having sufficient corrosion resistance, excellent formability, and excellent ridging resistance at the same time has not yet been well established in the state of the art.

Therefore, the present inventors conducted a bulge forming test which simulates a ventilation hood by using various kinds of ferritic stainless steel sheets (including those respectively corresponding to Patent Literature 1 through Patent Literature 3) which were manufactured by using different chemical compositions and different manufacturing methods. As a result, it was found that, since there may be a case where a steel sheet having a high elongation after fracture is inferior in terms of bulge forming capability to a steel sheet having a low elongation after fracture, the level of bulge forming capability does not necessarily depend on the level of elongation after fracture. Therefore, the bulge forming capability of the steel sheets which were used in the bulge forming test was evaluated in detail by drawing a forming limit diagram (FLD). As a result, it was found that bulge forming capability represented by a minimum value of maximum logarithmic strain at the forming limit, which is determined on the basis of an FLD, of 0.15 or more, or preferably 0.18 or more, is necessary in order to achieve good formability in the above-described bulge forming test which simulates a ventilation hood.

Subsequently, the present inventors conducted investigations regarding the reason why there may be a case where the level of bulge forming capability of a ferritic stainless steel sheet which is manufactured by using a conventional technique does not correspond to the level of elongation after fracture. As a result, the reason was found to be that a microstructure which is formed after cold-rolled-sheet annealing has been performed is a ferrite single-phase microstructure in which a large number of carbonitrides are homogeneously dispersed in all the cases of the conventional techniques in which box annealing or continuous annealing is performed. In the case where a steel sheet is subjected to forming, voids are formed in a microstructure as the amount of strain is increased, a crack is formed as a result of such voids combining with each other, and fracturing occurs eventually. Since the formation of such voids starts from carbonitrides in a metallographic structure, and since the microstructure of a ferritic stainless steel sheet which is manufactured by using a conventional technique is a ferrite single-phase microstructure in which a large number of carbonitrides are homogeneously dispersed, the formation of a very large number of voids starts in the whole metallographic structure. That is, in the case of a conventional technique, the formation of a crack tends to occur due to voids combining with each other. It was found that, from the reasons described above, although a high elongation after fracture is achieved in the case of, for example, a tensile test which involves uniaxial deformation, the combination of voids occurs in all the directions in the case of bulge forming which involves multiaxial stress and strain, which results in an increased tendency for fracturing to occur so that there may be a case where it is not possible to achieve sufficient bulge forming capability.

Therefore, the present inventors devised a technique in which, by performing cold rolling through the use of a commonly used method after performing hot-rolled-sheet annealing on a steel sheet having an appropriate chemical composition in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed, and by further performing cold-rolled-sheet annealing in a temperature range in which a ferrite single phase is formed, a ferrite single-phase microstructure is finally re-formed. It was found that, with this technique, it is possible to satisfy all the requirements, which are the targets of aspects of the present invention, that is, excellent bulge forming capability, elongation after fracture, average r-value, and ridging resistance.

Hereafter, aspects of the present invention will be described in detail on the basis of the obtained knowledge.

By performing hot-rolled-sheet annealing in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed and which is above a temperature range in which a ferrite single phase is formed, an austenite phase is formed in an amount of 3% to 20% in terms of area ratio in a hot-rolled-sheet annealing process. Almost all of this austenite phase transforms into a martensite phase in a cooling process following the hot-rolled-sheet annealing process. In the case where a dual-phase microstructure composed of a ferrite phase and a martensite phase is subjected to cold rolling and cold-rolled-sheet annealing, a martensite phase is decomposed into a ferrite phase and carbonitrides in the cold-rolled-sheet annealing process. As a result of such a microstructure change, a microstructure after cold-rolled-sheet annealing has been performed is composed of ferrite grains which are originally in a ferrite phase and ferrite grains which are formed as a result of the decomposition of a martensite phase. That is, since a large number of carbonitrides exist at the grain boundaries and within the ferrite grains which are formed as a result of the decomposition of a martensite phase, the whole metallographic structure is a mixed-grain microstructure composed of ferrite grains containing a very large number of carbonitrides at the grain boundaries and within the ferrite grains and ferrite grains containing a small number of carbonitrides. Since ferrite grains containing a large number of carbonitrides are relatively harder than ferrite grains containing a small number of carbonitrides, there is a difference in hardness among individual grains in a metallographic structure. It was found that, in the case where such a steel sheet is subjected to bulge forming, the formation of voids starts mainly from carbonitrides at the grain boundaries of ferrite grains containing a large number of carbonitrides and ferrite grains containing a small number of carbonitrides, while the number of formed voids is small in other portions. That is, in the steel according to aspects of the present invention, the number of formed voids is small in portions in which ferrite grains containing a large number of carbonitrides exist being connected to one another, in portions in which ferrite grains containing a small amount of carbonitrides exist being connected to one another, and within the ferrite grains. Therefore, the distance between voids is larger than in the case of a ferritic stainless steel sheet which is obtained by using a conventional technique, which results in cracking due to the combination of voids at the time when bulge forming is performed being less likely to occur. Therefore, it is possible to achieve such high bulge forming capability that the minimum value of maximum logarithmic strain at the forming limit, which is determined on the basis of an FLD, is 0.15 or more.

In addition, from the results of additional investigations conducted by the present inventors, it was found that it is necessary to appropriately control the C content and the N content in steel and a hot-rolled-sheet annealing temperature in order to realize the effects of aspects of the present invention. That is, it is necessary that the contents of C and N, which are austenite-forming chemical elements, be respectively at least 0.005% or more in order to form an austenite phase in an amount of 3% to 20% by performing hot-rolled-sheet annealing in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed. On the other hand, in the case where any one of the C content and the N content is more than 0.025%, the amount of an austenite phase formed when hot-rolled-sheet annealing is performed is excessively increased to more than 20%. As a result, there is an increase in the number of ferrite grains containing a large number of carbonitrides which are formed by performing subsequent cold-rolled-sheet annealing, which makes it impossible to achieve the specified bulge forming capability due to an increase in the area of grain boundaries of ferrite grains containing a large number of carbonitrides and ferrite grains containing a small number of carbonitrides from which the formation of voids starts when forming work is performed. Therefore, it is necessary that the upper limit of the C content and the upper limit of the N content both be 0.025%.

Regarding a hot-rolled-sheet annealing temperature, by performing annealing in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed, in particular, in a temperature range of 900° C. to 1100° C., it is possible to stably form a specified amount of austenite phase, and it is possible to achieve good surface quality without excessively increasing the grain size after cold-rolled-sheet annealing has been performed.

Moreover, it was found that it is possible to obtain advantageous effect regarding elongation after fracture, average r-value, and ridging resistance by performing hot-rolled-sheet annealing in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed, which is one of the technical features of aspects of the present invention, on steel having the C content and the N content described above. Although hot-rolled-sheet annealing is performed in a temperature range in which a ferrite single phase is formed in a conventional technique, since hot-rolled-sheet annealing is performed in a high temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed in the case of certain aspects of the present invention, the recrystallization and grain growth of a ferrite phase are promoted to a higher degree, which results in crystal grains growing to an appropriate size. With this, it is possible to realize the effect of increasing elongation after fracture and increasing average r-value due to the growth of an annealed texture being promoted to a higher degree. Here, elongation after fracture is also increased for the following reason. By decreasing the C content and the N content to a level which is recommended according to aspects of the present invention, since there is a decrease in the number of carbonitrides formed after cold-rolled-sheet annealing has been performed, the formation of voids and the combination of voids are inhibited when tensile deformation is applied. With this also, elongation after fracture is increased.

The reason why it is possible to realize an advantageous effect regarding ridging resistance is as follows. When an austenite phase is formed from a ferrite phase in a hot-rolled-sheet annealing process, the austenite phase has a crystal orientation which is different from that of the ferrite phase before annealing is performed. Moreover, a metallographic structure after hot-rolled-sheet annealing has been performed is a dual-phase microstructure composed of a martensite phase and a ferrite phase. Subsequently, since rolling strain is locally concentrated in ferrite phase grains which are interposed between martensite phase grains in a cold rolling process, misorientation occurs in ferrite phase grains. As a result of the occurrence of misorientation in ferrite phase grains, recrystallization is more likely to occur in a portion in which misorientation occurred in a subsequent cold-rolled-sheet annealing process. As a result, since the colony of a ferrite phase is effectively broken, it is possible to achieve excellent ridging resistance represented by a ridging height of 2.5 μm or less.

Consequently, it is necessary that the following conditions be satisfied in order to achieve all of sufficient bulge forming capability, elongation after fracture, average r-value, and ridging resistance. First, it is necessary that the chemical composition of steel have the C content and the N content with which an austenite phase is formed. On the basis of this condition, the C content and the N content is decreased within a range in which a specified amount of austenite phase is formed. After having performed hot-rolled-sheet annealing, on steel having such a chemical composition in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed, cold rolling and cold-rolled-sheet annealing are performed. It is necessary that, with this, a ferrite single-phase microstructure composed of ferrite grains containing a large number of carbonitrides and ferrite grains containing a small number of carbonitrides be formed.

Hereafter, the chemical composition of the ferritic stainless steel sheet according to aspects of the present invention will be described.

Hereinafter, “%” means “mass %”, unless otherwise noted.

C: 0.005% to 0.025%

C is effective for expanding a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed in a hot-rolled-sheet annealing process by promoting the formation of an austenite phase. It is necessary that the C content be 0.005% or more in order to obtain such an effect. However, in the case where the C content is more than 0.025%, since an excessive amount of austenite phase is formed in a hot-rolled-sheet annealing process, there is an excessive increase in the number of ferrite grains containing a large number of carbonitrides after cold-rolled-sheet annealing has been performed. As a result, since there is a decrease in the'distance between voids in a metallographic structure, fracturing caused by the combination of voids is more likely to occur when forming is performed, which makes it impossible to achieve sufficient bulge forming capability. Therefore, the C content is set to be in a range of 0.005% to 0.025%, or preferably 0.010% to 0.020%.

Si: 0.02% to 0.50%

Si is a chemical element which functions as a deoxidizing agent when molten steel is produced. It is necessary that the Si content be 0.02% or more in order to obtain such an effect. However, in the case where the Si content is more than 0.50%, there is an increase in rolling load due to an increase in the hardness of a steel sheet in a hot rolling process, and there is a deterioration in ductility after finish annealing has been performed. Therefore, the Si content is set to be in a range of 0.02% to 0.50%, preferably 0.10% to 0.35%, or more preferably 0.10% to 0.20%.

Mn: 0.55% to 1.00%

Mn is, like C, effective for expanding a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed in a hot-rolled-sheet annealing process by promoting the formation of an austenite phase. It is necessary that the Mn content be 0.55% or more in order to obtain such an effect. However, in the case where the Mn content is more than 1.00%, there is a deterioration in corrosion resistance due to an increase in the amount of MnS formed. Therefore, the Mn content is set to be in a range of 0.55% to 1.00%, preferably 0.60% to 0.90%, or more preferably, 0.75% to 0.85%.

P: 0.04% or less

Since P is a chemical element which promotes grain boundary fracturing due to grain boundary segregation, it is desirable that the P content be as small as possible, and the upper limit of the P content is set to be 0.04%, preferably 0.03% or less, or more preferably 0.01% or less.

S: 0.01% or less

S is a chemical element which deteriorates, for example, ductility and corrosion resistance by existing in the form of sulfide-based inclusions such as MnS, and such negative effects become marked particularly in the case where the S content is more than 0.01%. Therefore, it is desirable that the S content be as small as possible, and, in accordance with aspects of the present invention, the upper limit of the S content is set to be 0.01%, preferably 0.007% or less, or more preferably 0.005% or less.

Al: 0.001% to 0.10%

Al is, like Si, a chemical element which functions as a deoxidizing agent. It is necessary that the Al content be 0.001% or more in order to obtain such an effect. However, in the case where the Al content is more than 0.10%, there is a tendency for surface quality to deteriorate due to an increase in the amount of Al-based inclusions such as Al₂O₃. Therefore, the Al content is set to be in a range of 0.001% to 0.10%, preferably 0.001% to 0.07%, or more preferably 0.001% to 0.05%.

Cr: 15.5% to 18.0%

Cr is a chemical element which is effective for improving corrosion resistance by forming a passivation film on the surface of a steel sheet. It is necessary that the Cr content be 15.5% or more in order to obtain such an effect. However, in the case where the Cr content is more than 18.0%, since an insufficient amount of austenite phase is formed in hot-rolled-sheet annealing process, it is not possible to achieve the desired material properties. Therefore, the Cr content is set to be in a range of 15.5% to 18.0%, preferably 16.0% to 17.0%, or more preferably 16.0% to 16.5%.

Ni: 0.1% to 1.0%

Ni is a chemical element which improves corrosion resistance, and adding Ni is effective particularly in the case where a high level of corrosion resistance is required. In addition, Ni is effective for expanding a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed in a hot-rolled-sheet annealing process by promoting the formation of an austenite phase. Such effects become marked in the case where the Ni content is 0.1% or more. However, it is not desirable that the Ni content be more than 1.0%, because this results in a deterioration in formability. Therefore, in the case where Ni is added, the Ni content is set to be 0.1% to 1.0%, or preferably 0.1% to 0.3%.

N: 0.005% to 0.025%

N is, like C and Mn, is effective for expanding a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed in a hot-rolled-sheet annealing process by promoting the formation of an austenite phase. It is necessary that the N content be 0.005% or more in order to obtain such an effect. However, in the case where the N content is more than 0.025%, there is a significant deterioration in ductility, and an excessive number of ferrite grains containing a large number of carbonitrides are formed after cold-rolled-sheet annealing has been performed due to an excessive amount of austenite phase formed in a hot-rolled-sheet annealing process. As a result, since there is a decrease in the distance between voids in a metallographic structure, there is a tendency for fracturing caused by the combination of voids to occur when forming is performed, which makes it impossible to achieve sufficient bulge forming capability. Therefore, the N content is set to be in a range of 0.005% to 0.025%, or preferably 0.010% to 0.020%.

The remainder is Fe and inevitable impurities.

Although it is possible to realize the effects of aspects of the present invention with the chemical composition described above, the following chemical elements may be added in order to further improve manufacturability or material properties.

One, two, or more selected from Cu: 0.1% to 1.0%, V: 0.01% to 0.10%, Ti: 0.001% to 0.05%, Nb: 0.001% to 0.05%, Mo: 0.1% to 0.5%, and Co: 0.01% to 0.2%

Cu: 0.1% to 1.0%

Cu is a chemical element which improves corrosion resistance, and adding Cu is effective particularly in the case where a high level of corrosion resistance is required. In addition, Cu is effective for expanding a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed in a hot-rolled-sheet annealing process by promoting the formation of an austenite phase. Such effects become marked in the case where the Cu content is 0.1% or more. However, it is not preferable that the Cu content be more than 1.0%, because this may result in a deterioration in formability. Therefore, in the case where Cu is added, the Cu content is set to be 0.1% to 1.0%, or preferably in a range of 0.2% to 0.3%.

V: 0.01% to 0.10.%

V decreases the amounts of solid solute C and solid solute N by combining with C and N in steel. With this, there is an increase in average r-value. It is necessary that the V content be 0.01% or more in order to obtain such an effect. However, in the case where the V content is more than 0.10%, there is a deterioration in workability, and there is an increase in manufacturing costs. Therefore, in the case where V is added, the V content is set to be in a range of 0.01% to 0.10%, or preferably 0.02% to 0.08%.

Ti: 0.001% to 0.05% an Nb: 0.001% to 0.05%

Ti and Nb are, like V, chemical elements which have a high affinity for C and N and which is effective for improving workability after cold-rolled-sheet annealing has been performed by decreasing the amounts of solid solute C and solid solute N in a parent phase as a result of being precipitated in the form of carbides or nitrides in a hot rolling process. It is necessary that the Ti content be 0.001% or more and that the Nb content be 0.001% or more in order to realize such an effect. However, in the case where the Ti content is more than 0.05% or in the case where the Nb content is more than 0.05%, it is not possible to achieve good surface quality due to an excessive amount of TiN or NbC precipitated. Therefore, in the case where Ti is added, the Ti content is set to be in a range of 0.001% to 0.05%. and, in the case where Nb is added, the Nb content is set to be in a range of 0.001% to 0.05%. Ti content is preferably in a range of 0.003% to 0.03%, or more preferably 0.005% to 0.015%. Nb content is preferably in a range of 0.003% to 0.03%, or more preferably 0.005% to 0.015%.

Mo: 0.1% to 0.5%

Mo is a chemical element which improves corrosion resistance, and adding Mo is effective particularly in the case where a high level of corrosion resistance is required. Such an effect becomes marked in the case where the Mo content is 0.1% or more. However, it is not preferable that the Mo content be more than 0.5%, because this makes it impossible to achieve the desired material properties due to an insufficient amount of austenite phase being formed in a hot-rolled-sheet annealing process. Therefore, in the case where Mo is added, the Mo content is set to be 0.1% to 0.5%, or preferably in a range of 0.2% to 0.3%.

Co: 0.01% to 0.2%

Co is a chemical element which improves toughness. Such an effect is obtained in the case where the Co content is 0.01% or more. On the other hand, there is a deterioration in formability in the case where the Co content is more than 0.2%. Therefore, in the case where Co is added, the Co content is set to be in a range of 0.01% to 0.2%.

One, two, or more selected from Mg: 0.0002% to 0.0050%, Ca: 0.0002% to 0.0020%, B: 0.0002% to 0.0050%, and REM: 0.01% to 0.10%

Mg: 0.0002% to 0.0050%

Mg is a chemical element which is effective for improving hot workability. It is necessary that the Mg content be 0.0002% or more in order to obtain such an effect. However, in the case where the Mg content is more than 0.0050%, there is a deterioration in surface quality. Therefore, in the case where Mg is added, the Mg content is set to be in a range of 0.0002% to 0.0050%, preferably 0.0005% to 0.0035%, or more preferably 0.0005% to 0.0020%.

Ca: 0.0002% to 0.0020%

Ca is a constituent chemical element which is effective for preventing nozzle clogging which tends to occur due to the precipitation of inclusions in a continuous casting process. It is necessary that the Ca content be 0.0002% or more in order to obtain such an effect. However, in the case where the Ca content is more than 0.0020%, there is a deterioration in corrosion resistance due to the formation of CaS. Therefore, in the case where Ca is added, the Ca content is set to be in a range of 0.0002% to 0.0020%, preferably 0.0005% to 0.0015%, or more preferably 0.0005% to 0.0010%.

B: 0.0002% to 0.0050%

B is a chemical element which is effective for preventing secondary cold work embrittlement. It is necessary that the B content be 0.0002% or more in order to obtain such an effect. However, in the case where the B content is more than 0.0050%, there is a deterioration in hot workability. Therefore, in the case where B is added, the B content is set to be in a range of 0.0002% to 0.0050%, preferably 0.0005% to 0.0035%, or more preferably 0.0005% to 0.0020%.

REM: 0.01% to 0.10%

REM (rare earth metals) is a chemical element which improves oxidation resistance and which is particularly effective for improving the corrosion resistance of a welded zone by inhibiting the formation of an oxide film in the welded zone. It is necessary that the REM content be 0.01% or more in order to obtain such an effect. However, in the case where the REM content is more than 0.10%, there is a deterioration in manufacturability such as pickling capability in a cold-rolled-sheet annealing process. In addition, since REM is an expensive chemical element, it is not preferable that the REM content is excessively large, because this results in an increase in manufacturing costs. Therefore, in the case where REM is added, the REM content is set to be in a range of 0.01% to 0.10%, or preferably 0.01% to 0.05%.

Hereafter, the method for manufacturing the ferritic stainless steel sheet according to aspects of the present invention will be described. It is possible to obtain the ferritic stainless steel sheet according to aspects of the present invention by performing hot rolling on a steel slab having the chemical composition described above, by performing hot-rolled-sheet annealing in which the hot-rolled steel sheet is held in a temperature range of 900° C. to 1100° C. for 5 seconds to 15 minutes, by performing cold rolling, and by performing cold-rolled-sheet annealing in which the cold-rolled steel sheet is held in a temperature range of 800° C. to 900° C. for 5 seconds to 5 minutes.

First, after having prepared molten steel having the chemical composition described above by using a known method such as one using a converter, an electric furnace, or a vacuum melting furnace, a steel material (slab) is obtained by using a continuous casting method or an ingot casting-slabbing method. This slab is made into a hot-rolled steel sheet by performing hot rolling after having heated the slab at a temperature of 1100° C. to 1250° C. for 1 hour to 24 hours or by performing hot rolling directly on the slab as cast without heating the slab.

Subsequently, hot rolling is performed. It is preferable that coiling be performed at a coiling temperature of 500° C. or higher and 850° C. or lower. In the case where the coiling temperature is lower than 500° C., since a martensite phase is formed in a hot-rolled-sheet microstructure after coiling has been performed, recrystallization and grain growth are delayed in a subsequent hot-rolled-sheet annealing process. As a result, there is an increase in the number of fine grains in a hot-rolled and annealed sheet microstructure, and such fine grains are retained in a cold-rolled and annealed sheet microstructure, which may result in a deterioration in ductility after cold-rolled-sheet annealing has been performed. This is why it is not preferable that the coiling temperature be lower than 500° C. In the case where coiling is performed at a temperature of higher than 850° C., since there is an increase in grain size, there may be a case where surface deterioration in roughness occurs when press working is performed. Therefore, it is preferable that the coiling temperature be in a range of 500° C. to 850° C.

Hot-rolled-sheet annealing in which a hot-rolled steel sheet is held in a temperature range of 900° C. to 1100° C. for 5 seconds to 15 minutes

Subsequently, a hot-rolled-sheet annealing is performed in which a hot-rolled steel sheet is held in a temperature range of 900° C. to 1100° C. in which a dual phase composed of a ferrite phase and an austenite phase is formed for 5 seconds to 15 minutes.

A hot-rolled-sheet annealing process is a very important process for achieving excellent formability and ridging resistance in accordance with aspects of the present invention. In the case where the hot-rolled-sheet annealing temperature is lower than 900° C., since sufficient recrystallization does not occur, and since annealing is performed in a temperature range in which a ferrite single phase is formed, there may be a case where it is not possible to obtain the effect of aspects of the present invention which is obtained by performing annealing in a temperature range in which a dual phase is formed. On the other hand, in the case where the hot-rolled-sheet annealing temperature is higher than 1100° C., since there is a significant decrease in the amount of an austenite phase formed, there may be a case where it is not possible to achieve the specified ridging resistance. In the case where the annealing time is less than 5 seconds, since the formation of a sufficient amount of austenite phase or sufficient recrystallization of a ferrite phase does not occur even if annealing is performed at the specified temperature, there may be a case where it is not possible to achieve the desired formability. On the other hand, in the case where the annealing time is more than 15 minutes, since the concentration of C into an austenite phase is promoted, there is an excessive increase in the hardness of a martensite phase. As a result, since surface defects occur on the surface of a steel sheet in a subsequent cold rolling process due to an excessively hard martensite, there may be a case of a deterioration in surface quality after cold-rolled-sheet annealing has been performed. Therefore, hot-rolled-sheet annealing should be performed in a temperature range of 900° C. to 1100° C. for 5 seconds to 15 minutes, preferably in a temperature range of 920° C. to 1080° C. for 15 seconds to 5 minutes, or more preferably in a temperature range of 940° C. to 1040° C. for 30 seconds to 3 minutes.

Subsequently, after having performed pickling as needed, cold rolling is performed. It is preferable that cold rolling be performed with a rolling reduction of 50% or more from the viewpoint of elongation capability, bendability, press formability, and shape correction. In addition, in accordance with aspects of the present invention, a set of cold rolling and annealing thereafter may be repeated twice or more. Moreover, for example, grinding or polishing may be performed in order to further improve surface quality.

Cold-rolled-sheet annealing in which a cold-rolled steel sheet is held in a temperature range of 800° C. to 900° C. for 5 seconds to 5 minutes

Subsequently, cold-rolled-sheet annealing is performed. The cold-rolled-sheet annealing process is an important process for transforming a dual-phase microstructure composed of a ferrite phase and a martensite phase, which has been formed in a hot-rolled-sheet annealing process, into a ferrite single-phase microstructure. In the case where the cold-rolled-sheet annealing temperature is lower than 800° C., since sufficient recrystallization does not occur, it is not possible to achieve the specified formability. On the other hand, in the case where the cold-rolled-sheet annealing temperature is higher than 900° C., if this temperature is in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed for the treated steel chemical composition, since a martensite phase is formed after a cold-rolled-sheet annealing process, it is not possible to achieve the specified elongation after fracture or bulge forming capability due to an increase in the hardness of a steel sheet. In addition, even if this temperature is in a temperature range in which a ferrite single phase is formed for the treated steel chemical composition, it is not preferable that cold-rolled-sheet annealing be performed at this temperature from the viewpoint of surface quality, because this results in a decrease in the glossiness of a steel sheet due to a significant increase in crystal grain size. In the case where the annealing time is less than 5 seconds, even if annealing is performed at the specified temperature, since sufficient recrystallization of a ferrite phase does not occur, it is not possible to achieve the specified formability. It is not desirable that the annealing time be more than 5 minutes from the viewpoint of surface quality, because this results in a decrease in the glossiness of a steel sheet due to a significant increase in crystal grain size. Therefore, cold-rolled-sheet annealing should be performed in a temperature range of 800° C. to 900° C. for 5 seconds to 5 minutes, or preferably in a temperature range of 850° C. to 900° C. for 15 seconds to 3 minutes. BA annealing (bright annealing) may be performed in order to achieve a higher level of glossiness.

A product is obtained by further performing pickling as needed.

EXAMPLE 1

Hereafter, aspects of the present invention will be described in detail on the basis of examples.

Stainless steels having the chemical compositions given in Table 1 were prepared by using a small-size vacuum melting furnace having a capacity of 50 kg. Ingots of these steels were heated at a temperature of 1150° C. for one hour and then subjected to hot rolling to obtain hot-rolled steel sheets having a thickness of 3.5 mm. Subsequently, after having performed hot-rolled-sheet annealing on these hot-rolled steel sheets under the conditions given in Table 2, descaling involving a shot blasting treatment and pickling was performed on the surface. Moreover, after having performed cold rolling to obtain a thickness of 0.8 mm, cold-rolled-sheet annealing was performed under the conditions given in Table 2. Moreover, by performing a descaling treatment involving pickling, cold-rolled, pickled, and annealed steel sheets (ferritic stainless steel sheets) were obtained.

The cold-rolled, pickled, and annealed steel sheets (ferritic stainless steel sheets) obtained as described above were evaluated as follows.

(1) Evaluation of Bulge Forming Capability

By using a sample which had been taken from the cold-rolled, pickled, and annealed steel sheet and whose surface had been marked with scribed circles having a diameter of 5 mm so that the gauge length was 1 mm as a test piece, a forming limit diagram (FLD) was obtained by using a Nakajima method with a direction parallel to the rolling direction, a direction at an angle of 45° to the rolling direction, and a direction at a right angle to the rolling direction being respectively the directions of maximum logarithmic strain. By deriving the minimum value of maximum logarithmic strain at the forming limit from the obtained FLD, a case where the minimum value of maximum logarithmic strain was 0.15 or more was judged as satisfactory (◯), a case where the minimum value of maximum logarithmic strain was 0.18 or more was judged as particularly excellent (⊙), and a case where the minimum value of maximum logarithmic strain was less than 0.15 was judged as unsatisfactory (x).

(2) Evaluation of Ductility

By performing a tensile test in accordance with JIS Z 2241 on a JIS No. 13B tensile test piece taken from the cold-rolled, pickled, and annealed steel sheet (ferritic stainless steel sheet) in a direction at a right angle to the rolling direction, and by determining elongation after fracture, a case where the elongation after fracture was 28% or more was judged as satisfactory (◯), a case where the elongation after fracture was 30% or more was judged as particularly excellent (⊙), and a case where the elongation after fracture was less than 28% was judged as unsatisfactory (x).

(3) Evaluation of Average r-Value

By stopping a tensile test in accordance with JIS Z 2411 when a strain of 15% was applied to JIS No. 13B tensile test pieces taken from the cold-rolled, pickled, and annealed steel sheet (ferritic stainless steel sheet) respectively in a direction (L-direction) parallel to the rolling direction, a direction (D-direction) at 45° to the rolling direction, and a direction (C-direction) at a right angle to the rolling direction, and by determining the r-value of each of the three directions, an average r-value (=(r_(L)+2r_(D)+r_(c))/4) was calculated. Here, r_(L), r_(D), and rc respectively denote the r-values in the L-direction, the D-direction, and the C-direction respectively. A case of an average r value of 0.75 or more was judged as satisfactory (◯), and a case of an average r-value of less than 0.75 was judged as unsatisfactory (x).

(4) Evaluation of Ridging Resistance

After having applied a tensile strain of 20% to a JIS No. 5 tensile test piece which had been taken from the cold-rolled, pickled, and annealed steel sheet (ferritic stainless steel sheet) in a direction parallel to the rolling direction and whose surface had been polished by using #600 emery paper, arithmetic average waviness (Wa) prescribed in JIS B 0601 (2001) in a direction at a right angle to the rolling direction was determined in the polished surface of the central portion of the parallel part of the test piece by using a surface roughness meter with a determination length of 16 mm, a wavelength of a high-frequency cutoff filter of 0.8 mm, a wavelength of a low-frequency cutoff filter of 8 mm. A case where the arithmetic average waviness (Wa) was 2.5 μm or less was judged as satisfactory (◯), and a case where the arithmetic average waviness (Wa) was more than 2.5 μm was judged as unsatisfactory (x).

(5) Evaluation of Corrosion Resistance

A salt spray cyclic corrosion test prescribed in JIS H 8502 was performed on a test piece of 60 mm×100 mm which had been taken from the cold-rolled, pickled, and annealed steel sheet, whose surface had been polished by using #600 emery paper, and whose end surfaces were sealed. A salt spray cyclic corrosion test was performed 8 cycles, where a unit cycle consists of salt spraying (5-mass %-NaCl, 35° C., spraying for 2 hours), drying (60° C., 4 hours, relative humidity 40%), and wetting (50° C., 2 hours, relative humidity ≥95%).

By taking the photograph of the surface of the test piece after 8 cycles of the salt spray cyclic corrosion test had been performed, by determining the rust area on one side of the test piece by performing image analysis, a rust area ratio ((rust area in the test piece)/(total area of the test piece)×100 [%]) was calculated as the ratio of the rust area to the total area of the test piece. A case where the rust area ratio was 10% or less was judged as a satisfactory case of particularly excellent corrosion resistance (⊙), a case where the rust area ratio was more than 10% and 25% or less was judged as satisfactory (◯), and a case where the rust area ratio was more than 25% was judged as unsatisfactory (x).

The evaluation results are given in Table 2 along with the hot-rolled-sheet annealing conditions and the cold-rolled-sheet annealing conditions.

TABLE 1 mass % Steel No. C Si Mn P S Cr Al N Ni Other Note S1 0.006 0.15 0.77 0.034 0.009 16.14 0.004 0.011 0.19 Example S2 0.010 0.19 0.80 0.020 0.003 16.88 0.003 0.008 0.22 Example S3 0.015 0.14 0.79 0.034 0.003 16.16 0.004 0.016 0.11 Example S4 0.019 0.18 0.74 0.023 0.004 16.92 0.004 0.014 0.27 Example S5 0.010 0.03 0.83 0.008 0.002 16.45 0.005 0.016 0.31 Example S6 0.009 0.50 0.79 0.029 0.010 16.47 0.002 0.018 0.16 Example S7 0.007 0.12 0.55 0.039 0.009 16.84 0.001 0.019 0.13 Example S8 0.012 0.25 0.98 0.033 0.002 16.74 0.003 0.015 0.20 Example S9 0.019 0.29 0.75 0.015 0.007 15.57 0.012 0.016 0.39 Example S10 0.013 0.15 0.84 0.027 0.010 17.80 0.009 0.009 0.17 Example S11 0.008 0.20 0.61 0.022 0.008 17.23 0.005 0.015 0.29 Example S12 0.009 0.19 0.70 0.023 0.009 17.02 0.003 0.006 0.37 Example S13 0.014 0.24 0.77 0.034 0.010 16.15 0.004 0.017 0.24 Example S14 0.014 0.25 0.64 0.016 0.002 17.32 0.003 0.023 0.38 Example S15 0.019 0.29 0.90 0.007 0.008 16.64 0.004 0.009 0.26 Ti: 0.012, Nb: 0.014 Example S16 0.007 0.27 0.72 0.022 0.004 17.04 0.005 0.009 0.21 V: 0.08, Ti: 0.012, Nb: 0.028 Example S17 0.008 0.14 0.62 0.036 0.007 16.03 0.003 0.013 0.53 Example S18 0.011 0.11 0.85 0.010 0.005 16.85 0.014 0.008 0.36 Cu: 0.4 Example S19 0.018 0.31 0.61 0.033 0.004 16.63 0.019 0.012 0.20 Mo: 0.3 Example S20 0.010 0.20 0.87 0.032 0.008 17.30 0.002 0.011 0.39 Co: 0.2 Example S21 0.005 0.27 0.68 0.008 0.005 16.32 0.003 0.018 0.15 Mg: 0.0016 Example S22 0.011 0.22 0.86 0.026 0.009 16.81 0.003 0.014 0.18 Ca: 0.0012 Example S23 0.010 0.23 0.77 0.008 0.007 16.41 0.003 0.015 0.33 B: 0.0011 Example S24 0.012 0.31 0.90 0.016 0.007 17.06 0.077 0.009 0.36 REM: 0.05 Example S25 0.002 0.17 0.78 0.019 0.004 16.34 0.004 0.017 0.33 Comparative Example S26 0.027 0.15 0.83 0.027 0.005 16.79 0.002 0.014 0.22 Comparative Example S27 0.014 0.58 0.69 0.031 0.006 16.46 0.003 0.012 0.16 Comparative Example S28 0.009 0.18 0.17 0.034 0.004 16.38 0.005 0.016 0.28 Comparative Example S29 0.018 0.35 1.06 0.035 0.006 16.87 0.004 0.017 0.28 Comparative Example S30 0.011 0.20 0.64 0.028 0.003 15.30 0.005 0.013 0.10 Comparative Example S31 0.016 0.14 0.79 0.030 0.005 18.24 0.004 0.015 0.38 Comparative Example S32 0.016 0.26 0.72 0.021 0.004 16.33 0.003 0.003 0.22 Comparative Example S33 0.014 0.19 0.80 0.027 0.006 16.26 0.003 0.028 0.24 Comparative Example Annotation: An underlined portion indicates a value out of the range according to the present invention.

TABLE 2 Hot-rolled-sheet Cold-rolled-sheet Annealing Annealing Elongation Bulge Steel Temperature Time Temperature Time After Average Ridging Corrosion Forming No. No. [° C.] [s] [° C.] [s] fracture r-value Height Resistance Capability Note 1 S1 1004 62 852 61 ⊙ ◯ ◯ ◯ ⊙ Example 2 S2  995 60 853 62 ⊙ ◯ ◯ ◯ ⊙ Example 3 S3 1002 61 850 62 ⊙ ◯ ◯ ◯ ⊙ Example 4 S4 1008 62 854 61 ⊙ ◯ ◯ ◯ ⊙ Example 5 S5 1009 60 849 60 ⊙ ◯ ◯ ◯ ⊙ Example 6 S6 1002 62 848 62 ◯ ◯ ◯ ◯ ◯ Example 7 S7 1000 61 847 61 ⊙ ◯ ◯ ◯ ⊙ Example 8 S8 1004 62 851 60 ⊙ ◯ ◯ ◯ ⊙ Example 9 S9  993 60 852 62 ⊙ ◯ ◯ ◯ ⊙ Example 10 S10 1008 61 849 60 ⊙ ◯ ◯ ⊙ ⊙ Example 11 S11  991 61 852 60 ⊙ ◯ ◯ ◯ ⊙ Example 12 S12 1005 60 845 62 ⊙ ◯ ◯ ◯ ⊙ Example 13 S13 1009 60 847 62 ⊙ ◯ ◯ ◯ ⊙ Example 14 S14 1002 60 851 60 ◯ ◯ ◯ ◯ ◯ Example 15 S15 1008 61 847 62 ⊙ ◯ ◯ ◯ ⊙ Example 16 S16  995 62 850 60 ⊙ ◯ ◯ ◯ ⊙ Example 17 S17  993 61 851 62 ⊙ ◯ ◯ ⊙ ⊙ Example 18 S18  995 61 845 60 ⊙ ◯ ◯ ⊙ ⊙ Example 19 S19 1000 61 848 60 ⊙ ◯ ◯ ⊙ ⊙ Example 20 S20 1003 62 848 61 ⊙ ◯ ◯ ◯ ⊙ Example 21 S21 1000 61 846 61 ⊙ ◯ ◯ ◯ ⊙ Example 22 S22  995 60 848 62 ⊙ ◯ ◯ ◯ ⊙ Example 23 S23  996 60 851 60 ⊙ ◯ ◯ ◯ ⊙ Example 24 S24  993 62 849 62 ⊙ ◯ ◯ ◯ ⊙ Example 25 S3  901 61 850 61 ⊙ ◯ ◯ ◯ ⊙ Example 26 S3 1098 60 853 62 ⊙ ◯ ◯ ◯ ⊙ Example 27 S3 1002  5 850 60 ⊙ ◯ ◯ ◯ ⊙ Example 28 S3 1000 900  852 61 ⊙ ◯ ◯ ◯ ⊙ Example 29 S16  998 61 801 60 ⊙ ◯ ◯ ◯ ⊙ Example 30 S16 1001 60 900 62 ⊙ ◯ ◯ ◯ ⊙ Example 31 S16 1002 60 845  5 ⊙ ◯ ◯ ◯ ⊙ Example 32 S16 1000 60 844 898  ⊙ ◯ ◯ ◯ ⊙ Example 33 S25 1003 62 841 60 ◯ ◯ X ◯ X Comparative Example 34 S26 1003 63 840 61 X ◯ ◯ ◯ ◯ Comparative Example 35 S27 1001 60 843 62 X ◯ ◯ ◯ ◯ Comparative Example 36 S28 1005 59 842 62 ◯ ◯ X ◯ X Comparative Example 37 S29 999 63 844 60 ◯ ◯ ◯ X ◯ Comparative Example 38 S30 1000 60 843 59 ◯ ◯ ◯ X ◯ Comparative Example 39 S31 1004 58 838 63 ◯ ◯ X ◯ X Comparative Example 40 S32 1002 60 842 58 ◯ ◯ X ◯ X Comparative Example 41 S33  998 63 844 59 X ◯ ◯ X ◯ Comparative Example 42 S30  887 63 840 60 X X X ◯ X Comparative Example 43 S30 1121 61 841 60 ◯ ◯ X ◯ ◯ Comparative Example 44 S30 1005  3 844 63 X X ◯ ◯ X Comparative Example 45 S30 1002 63 786 60 X X ◯ ◯ X Comparative Example 46 S30 1000 61 917 59 X ◯ ◯ ◯ X Comparative Example 47 S30 1006 60 845  2 X ◯ ◯ ◯ X Comparative Example Annotation: An underlined portion indicates a value out of the range according to the present invention.

In the case of Nos. 1 through 32 (steels S1 through S24), which satisfied the requirements regarding the range of steel chemical composition according to aspects of the present invention, since the elongation after fracture was 28% or more, the average r-value was 0.75 or more, the ridging height was 2.5 μm or less, the rust area ratio of the surface of any of the test pieces after 8 cycles of a salt spray cyclic corrosion test had been performed was 25% or less in the evaluation of corrosion resistance, and the minimum value of maximum logarithmic strain at the forming limit, which was determined on the basis of an FLD, was 0.15 or more in the evaluation of bulge forming capability, it is clarified that excellent formability, corrosion resistance, and ridging resistance were achieved.

In particular, in the case of No. 10 (steel No. S10) where the Cr content was 17.80%, in the case of No. 17 (steel No. S17) where the Ni content was 0.4%, in the case of No. 18 (steel No. S18) where the Cu content was 0.4%, and in the case of No. 19 (steel No. S19) where the Mo content was 0.3%, since the rust area ratio after a salt spray cyclic corrosion test had been performed was 10% or less (⊙), it is clarified that there was an improvement in corrosion resistance to a higher level.

On the other hand, in the case of No. 38 (steel No. S30) where the Cr content was below the range according to aspects of the present invention, although the specified formability and ridging resistance were achieved, it was not possible to achieve the specified corrosion resistance due to insufficient Cr content.

In the case of No. 39 (steel No. S31) where the Cr content was above the range according to aspects of the present invention, although sufficient corrosion resistance was achieved, since an austenite phase was not formed in the hot-rolled-sheet annealing process due to excessive Cr content, it was not possible to achieve the specified ridging resistance. Moreover, since it was not possible to form a cold-rolled and annealed sheet microstructure composed of ferrite grains containing a large number of carbonitrides and ferrite grains containing a small number of carbonitrides, which ware formed by performing hot-rolled-sheet annealing in a temperature range in which a dual phase is formed, it was not possible to achieve the specified bulge forming capability.

In the case of No. 33 (steel No. S25) where the C content was below the range according to aspects of the present invention, although the specified elongation after fracture and average r-value were achieved, since an austenite phase was not formed in a hot-rolled-sheet annealing process due to insufficient austenite-forming capability, it was not possible to achieve the specified ridging resistance or bulge forming capability. In contrast, in the case of No. 34 (steel No. S26) where the C content was above the range according to aspects of the present invention, although the specified ridging resistance and bulge forming capability were achieved, since there was a decrease in elongation due to an increase in the hardness of the steel sheet, it was not possible to achieve the specified elongation after fracture.

In the case of No. 27 (steel No. S27) where the Si content was above the range according to aspects of the present invention, since there was an increase in the hardness of the steel sheet due to excessive Si content, it was not possible to achieve the specified elongation after fracture.

In the case of No. 36 (steel No. S28) where the Mn content below than the range according to aspects of the present invention, although the specified elongation after fracture and average r-value were achieved, since an austenite phase was not formed in the hot-rolled-sheet annealing process due to insufficient austenite-forming capability, it was not possible to achieve the specified ridging resistance or bulge forming capability. In contrast, in the case of No. 37 (steel No. S29) where the Mn content was above the range according to aspects of the present invention, since a large amount of MnS was formed in the microstructure, it was not possible to achieve the specified corrosion resistance.

In the case of No. 40 (steel No. S32) where the N content was below the range according to aspects of the present invention, although the specified elongation after fracture and average r-value were achieved, since an austenite phase was not formed in the hot-rolled-sheet annealing process due to insufficient austenite-forming capability, it was not possible to achieve the specified ridging resistance or bulge forming capability. In contrast, in the case of No. 41 (steel No. S33) where the N content was above the range according to aspects of the present invention, although the specified ridging resistance and bulge forming capability were achieved, since there was an increase in the hardness of the steel sheet, it was not possible to achieve the specified elongation after fracture. Moreover, since sensitization occurred due to a large amount of Cr nitrides precipitated in the microstructure, it was not possible to achieve the specified corrosion resistance.

In the case of Nos. 42 through 47, investigations regarding the influences of the conditions of hot-rolled-sheet annealing and cold-rolled-sheet annealing on formability and ridging resistance were conducted by using steel S30, which was not possible to achieve the specified corrosion resistance due to insufficient Cr content although the specified formability and ridging resistance were achieved. In the case of No. 42 where the hot-rolled-sheet annealing temperature was below the range according to aspects of the present invention, since an austenite phase was not formed due to the hot-rolled-sheet annealing temperature being in a temperature range in which a ferrite single phase was formed, it was not possible to achieve the specified ridging resistance or bulge forming capability. Also, since sufficient recrystallization did not occur, it was not possible to achieve the specified elongation after fracture or average r-value. In the case of No. 43 where the hot-rolled-sheet annealing temperature was above the range according to aspects of the present invention, since there was a decrease in the amount of an austenite phase formed, it was not possible to achieve the specified ridging resistance. In the case of No. 44 where the hot-rolled-sheet annealing time was below the range according to aspects of the present invention, since a sufficient amount of austenite phase was not formed, and since sufficient recrystallization did not occur, it was not possible to achieve the specified elongation after fracture, average r-value, or bulge forming capability. In the case of No. 45 and No. 47 where the cold-rolled-sheet annealing temperature was below the range according to aspects of the present invention or the cold-rolled-sheet annealing time was less than the range according to aspects of the present invention, since a martensite phase formed in the hot-rolled-sheet annealing process was retained, and since sufficient recrystallization did not occur, it was not possible to achieve the specified elongation after fracture or bulge forming capability. In the case of No. 46 where the cold-rolled-sheet annealing temperature was above the range according to aspects of the present invention, since there was an increase in the hardness of the steel sheet due to a martensite phase being formed as a result of the cold-rolled-sheet annealing temperature being in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase was formed, it was not possible to achieve the specified elongation after fracture or bulge forming capability.

INDUSTRIAL APPLICABILITY

The ferritic stainless steel sheet obtained by using aspects of the present invention can particularly suitably be used in applications in which products manufactured by performing press forming involving mainly bulge forming are required such as kitchen appliances and eating utensils. 

1. A ferritic stainless steel sheet having a chemical composition containing, by mass %, C: 0.005% to 0.025%, Si: 0.02% to 0.50%, Mn: 0.55% to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001% to 0.10%, Cr: 15.5% to 18.0%, Ni: 0.1% to 1.0%, N: 0.005% to 0.025%, and the balance being Fe and inevitable impurities, an elongation after fracture of 28% or more, an average r-value of 0.75 or more, and a minimum value of maximum logarithmic strain at the forming limit, which is determined on the basis of a forming limit diagram (FLD), of 0.15 or more.
 2. The ferritic stainless steel sheet according to claim 1, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from Cu: 0.1% to 1.0%, V: 0.01% to 0.10%, Ti: 0.001% to 0.05%, Nb: 0.001% to 0.05%, Mo: 0.1% to 0.5%, and Co: 0.01% to 0.2%.
 3. The ferritic stainless steel sheet according to claim 1, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from Mg: 0.0002% to 0.0050%, Ca: 0.0002% to 0.0020%, B: 0.0002% to 0.0050%, and REM: 0.01% to 0.10%.
 4. The ferritic stainless steel sheet according to claim 2, the steel sheet having the chemical composition further containing, by mass %, one, two or more selected from Mg: 0.0002% to 0.0050% Ca: 0.0002% to 0.0020%, B: 0.0002% to 0.0050%, and REM: 0.01% to 0.10%.
 5. A method for manufacturing the ferritic stainless steel sheet according to claim 1, the method comprising performing hot rolling on a steel slab, performing annealing in which the hot-rolled steel sheet is held in a temperature range of 900° C. to 1100° C. for 5 seconds to 15 minutes, subsequently performing cold rolling, and performing annealing in which the cold-rolled steel sheet is held in a temperature range of 800° C. to 900° C. for 5 seconds to 5 minutes.
 6. A method for manufacturing the ferritic stainless steel sheet according to claim 2, the method comprising performing hot rolling on a steel slab, performing annealing in which the hot-rolled steel sheet is held in a temperature range of 900° C. to 1100° C. for 5 seconds to 15 minutes, subsequently performing cold rolling, and performing annealing in which the cold-rolled steel sheet is held in a temperature range of 800° C. to 900° C. for 5 seconds to 5 minutes.
 7. A method for manufacturing the ferritic stainless steel sheet according to claim 3, the method comprising performing hot rolling on a steel slab, performing annealing in which the hot-rolled steel sheet is held in a temperature range of 900° C. to 1100° C. for 5 seconds to 15 minutes, subsequently performing cold rolling, and performing annealing in which the cold-rolled steel sheet is held in a temperature range of 800° C. to 900° C. for 5 seconds to 5 minutes.
 8. A method for manufacturing the ferritic stainless steel sheet according to claim 4, the method comprising performing hot rolling on a steel slab, performing annealing in which the hot-rolled steel sheet is held in a temperature range of 900° C. to 1100° C. for 5 seconds to 15 minutes, subsequently performing cold rolling, and performing annealing in which the cold-rolled steel sheet is held in a temperature range of 800° C. to 900° C. for 5 seconds to 5 minutes. 